Metallization of nonmetallic surfaces



Patented May 26, 1953 METALLIZATION OF NONMETALLIC SURFACES CyrilFrancis Drake, London, England, assignor to International StandardElectricCorporation, New York, N. Y;, a corporation of Delaware NoDrawing. Application April 22, 1948, Serial No. 22,700. In Great BritainApril 24, 1947 1 This invention relates to the metallisation ofnon-metallic surfaces and more particularly to a process of metallisingsuch surfaces whereby only a desired area of asurface may be coated withmetal.

The process of silvering glass by immersing the glass in an ammoniacalsolution of silver nitrate, it may be after having treated the glasswith an activating agent such as a solution of stannous chloride inhydrochloric acid, and thenadding. a reducing agent to the solution, iswell 'known.' This process suffers from'the disadvantages that silver isdeposited upon areas other than those required to be silvered so that itcannot be used for silvering only a portion of the surface immersed inthe solution, and, even if the deposit which occurs upon the sides ofthe vessel and other unwanted parts can be recovered (w'hidhis notalwayspossible to secure), a considerably larger amount of silver saltmust be used than is necessary to form a coating of given thickness on aglass of given area. In addition there is formed a considerable depositof silver in the solution itself which later settles out.

It is the object of the present invention to provide a process ofmetallising that does not suffer. from these disadvantages and has inconsequence a wider field of application than the known method.

-According to the present invention a process of metallisingnon-metallic surfaces (other than hydrocarbon polymers) by reduction ofa metal salt in solution is characterised in that the reduction of themetal salt in solution is effected in the presence of an anionic surfaceactive agent.

The invention finds its principal application in the silvering ofnon-metallic surfaces and will accordingly be explained in relationthereto.

In carrying out the invention in relation to the silvering ofnon-metallic surfaces, an anionic surface active agent is added to anammoniacal solution of silver nitrate. The preferred surface activeagent is a wetting compound in the form of an alkali metal oralkaline-earth-metal salt of a sulphonated long chain fatty alcohol;specifically the sodium salt of sulphonated lauryl alcohol has beenfound satisfactory and the proportion added was 0.05% by weight of thesolution. On now adding a reducing agent such as formaldehyde, noapparent change takes place in the solution.

Metallic silver is formed in the same way as in the absence of thesurface active agent but the particles of silver remain of such smalldimensions that the liquid is transparent and the silver 7 Claims. (01.117-35) 2- remains in suspension. Since the silver particles produced bythe reduction of a silver salt carry a, negative charge, it is necessaryfor the production of such a result that the surface active agent shouldbe anionic. The proportion of surface active agent added should not beso great that the silver will be permanently held in suspension nor thatthe silverwhen deposited will be discoloured. Thus in the case of thesodium salt of sulphonated lauryl alcohol the proportion should notexceed 0.2% by weight.

In order that a non-metallic surface shall be silvered on being immersedin the liquid it is necessary that the surface shall have been activatedin known manner, e. g. by immersion in a solution of stannous chloridein hydrochloric acid. If a glass surface so activated be exposed to theliquid above mentioned containing silver in suspension, the silver isdeposited immediately upon the activated surface and forms an'adherentmirror coating.

The surfaces of other non-metallic substances may be equally wellsilvered in this manner, provided that the surface has polar groupspresent. Thus surfaces of glass, mica, quartz and some synthetic resinssuch as those of the acrylic acid group may be silvered in this way, butadherent deposits are not formed on the surface of hydrocarbon polymerssuch as polystyrene or solid polymers or ethylene.

It is clear that the activated surface to be silvered can be placed inthe ammoniacal solution of silver nitrate containing the surface activeagent either before or after the reducing agent has been added thereto.It is, however, generally convenient to add the reducing agent to thesolution before exposing the surface to be silvered thereto. It is clearalso that it is not necessary to add all at once an amount of reducingagent sufficient to reduce all the silver salt in solution, but that itis possible to add first only sufiicient formaldehyde to precipitate thesilver required to silver a certain number of surfaces and then toexpose those surfaces to the liquid, after which a further amount offormaldehyde may be added and further surfaces silvered.

Since silver is only deposited upon areas that have been activated theinvention can be used for silvering only a required area on a surface.Thus electrodes of any desired shape and area can be formed upon platesof quartz piezo-electric crystals by activating only the parts of thequartz desired to be silvered.

The thickness of silver coating obtained can be varied from atransparent coating upwards to any desired thickness according to thetime of immersion, though it is obviously desirable if very thickcoatings are required, e. g. sufficient for solderin or connecting lead,to immerse the surface being silvered in more than one successive bath.

The present invention provides the only means known to us of uniformlysilvering the interior surface of a narrow core or long lengthcylindrical tube of glass or similar material. By washing the interiorsurface of the tube with is 5.01 tion of stannous chloride inhydrochloric acidand then drawing through the tube a solutioncorrtaining finely divided silver in suspension as described above, auniform and adherent silver deposit is formed on the interior surface ofthe tube only.

What is claimed is:

1. Process for producing an adherent coating of metallic silver upon asurface of an article formed of a non-metallic substance chosen from theclass consisting of glass, mica, quartz, and synthetic resins havingpolar groups present in the molecule, that comprises applying thenremoving, a solution of stannous chloride hydrochloric acid to thesurface to be coated, then treating the surface with an ammoniacalsolution of silver nitrate containing an anionic surface active agent inthe presence of a reducing agent.

'2. Process as claimed in claim 1 wherein the surface active agent is asubstance chosen vfrom the class consisting of the alkali-metal saltsand the alkaline-\earth-metal salts of a sulphonated long chain fattyalcohol.

,3. Process as claimed in claim ,1 wherein the surface active agent isthe sodium salt of sulphonated lauryl alcohol.

.4.- Process of silvering a surface of an electrical insulating materialhaving polar groups present which comprises activating the surface bytreatment with a solution of stannous chloride in hydrochloric acid andthereafter immersing the surface in a liquid containing silver insuspension and an anionic surface active agent.

5. Process of silvering a given area of a nonmetallic surface Of thekind thathaspolar groups 'on :the surface which comprises treating thatgiven area with a solution of stannous chloride in hydrochloric acid toactivate it, then treating it with an ammoniacal solution of silvernitrate med thereto a small percentage of the sadium salt of sulphonatedlauryl alcohol together withformaldehyde in an amount sufllcient toreduce the silver nitrate to silver.

6. Pmocess-of rsilvering the interior surface 01 a glass tube thatcomprises activating the said suniace by treatment with a hydrochloricacid solution of stannous chloride and thereafter drawing through thesaid tube an ammoniacal solution of silver nitrate containing a reducingagent and a small percentage of an anionic surface active agent of theclass consisting of alkali metal and alkaline earth-metal salts of's'ulionaied long-chain aliphatic alcohols.

7. Process according to claim 3, in which the sodium salt ,ofsulphonated lauryl alcohol comprises'0.0 5% to 0.2% by'weight oftheammoniacal solution.

CYRIL FRANCIS References Cited in the file .9! this UNITED cram mums IDate

1. PROCESS FOR PRODUCING AN ADHERENT COATING OF METALLIC SILVER UPON ASURFACE OF AN ARTICLE FORMED OF A NON-METALLIC SUBSTANCE CHOSEN FROM THECLASS CONSISTING OF GLASS, MICA, QUARTZ, AND SYNTHETIC RESINS HAVINGPOLAR GROUPS PRESENT IN THE MOLECULE, THAT COMPRISES APPLYING, THENREMOVING, A SOLUTION OF STANNOUS CHLORIDE IN HYDROCHLORIC ACID TO THESURFACE TO BE COATED, THEN TREATING THE SURFACE WITH AN AMMONIACALSOLUTION OF SILVER NITRATE CONTAINING AN ANIONIC SURFACE ACTIVE AGENT INTHE PRESENCE OF A REDUCING AGENT.